Development of the vertebrate musculoskeletal system requires the coordinated morphogenesis of muscle, muscle connective tissue, tendon, and skeleton. In the limb, muscle derives from migratory precursors originating from the somites, while the muscle connective tissue, tendons and skeletal elements derive from the mesoderm of the emerging lateral-plate derived limb bud. As the muscle precursors migrate into the limb, they must differentiate into myofibers, become correctly patterned into distinct anataomical muscles, and be assemebled into a functional musculoskeleton. How the over 40 limb muscles are patterned and muscle and muscle connective tissue morphogenesis coordinated is the subject of this proposal. Classical studies had suggested that lateral plate, limb mesodermal signals are important patterning muscle. However, neither the molecular nature of the signal nor the tissue producing it was known. Recently we have identified a population of lateral plate, limb mesodermal cells that expresses the transcription factor Tcf4, a downstream effector of the Wnt/beta-catenin signaling pathway, and that is critical for muscle patterning. Functional studies in the chick suggest that Tcf4-expressing cells establish a prepattern in the limb mesoderm that determines where muscle precursors differentiate and thus where individual muscles will form and the ultimate limb muscle pattern. In addition, preliminary studies indicate that Tcf4-expressing cells are the precursors of the muscle connective tissue, a tissue of fundamental importance to the form and function of the musculoskeleton and whose development has been largely unstudied because of the lack of early molecular markers. We propose to test genetically in the mouse the role of limb mseodermal Tcf4- expressing cells and Wnt/beta-catenin signaling in determining the pattern of limb muscles. In addition, we will determine whether Tcf4-expressing cells are precursors necessary for formation of muscle connective tissue. These experiments will provide important insights into the normal development of muscle and muscle connective tissue and the role of Wnt/beta-catenin signaling in regulating their development. Disruptions in the development of muscle and muscle connective tissues can result in severe musculoskeltal disorders, such as Duchenne's muscular dystrophy and Ullrich congenital muscular dystrophy. Results from our experiments will give us important new insights into the etiology of these diseases.